Have you ever wondered how Smithsonian Gardens’ horticulturists choose the plants in their gardens?  Janet Draper, Horticulturist in the Mary Livingston Ripley Garden, highlights her favorite plants for creating a garden with year-round interest. Nothing is off limits for this self-professed plant-aholic! Janet shares what plants she relies on when creating her exuberant displays. 

Resources: 

Designing Gardens – Foliage First     

Sunny Woody Layer   

Spirea thunbergia ‘Ogon’ – “Golden Bridalwreath Spirea.”  Native to China and Japan.  USDA Plant Hardiness Zones 4-8, full sun to light shade, average soils.  This old-fashioned stalwart got a color upgrade—displaying light, airy white flowers as early as March, followed by the emergence of dainty golden foliage that persists until fall when it takes on a lovely golden hue, before foliage drop.  All winter the delicate skeleton is exposed.  Maintenance is easy.  You can totally leave it alone and it gets taller and wider with time, or you can go in right after it blooms and remove the older stems to the ground, thus keeping a tighter mound of golden foliage.    

Yucca family!  All need full sun and well–drained soils.  All are drought–tolerant and evergreen.    

  • Yucca filamentosa – Adam’s Needle.  Southeast U.S. native (South Carolina to Florida, west to Mississippi), hardy in Zones 4-10. Selections include golden variegated ‘Color Guard’ and ‘Gold Sword.’  
  • Yucca recurvifolia – Curl Leaf Yucca.  USDA zones 7-9.  3-6’ tall.    
  • Yucca rostrata – Beaked Yucca.  Zones 6-10 (perhaps down to Zone 5).    

Physocarpusopulifolius – Ninebark.  Native to eastern North America from Quebec down to Tennessee.  Member of the Rose family.  In recent years breeders have worked with the wild 10-12’ sprawling mass and shrunk it down to fit smaller gardens and made selections ranging from bright golden to nearly black foliage.  Naturally found along stream banks, this tough plant is extremely adaptable from Zones 2-8 in full to partial sun and average to moist soils.  Cultivars include Diablo, ‘Amber Jubilee,‘ ‘Coppertina,‘ ‘Dart’s Gold,‘ ‘Little Devil,‘ and ‘Summer Wine.‘  

Woody Shade Lovers  

Edgeworthia chrysantha – Paper Bush or Chinese Paperbush.  Native to China and Himalayan region, Zones 7-9.  Reaches 8’ tall by 10’ wide.  Full sun to light shade.  Average, well–drained soils.    

Aucuba japonica – “Aucuba,” ‘Gold Dust.”  Light to full shade, average to dry soils.  Broad leaf evergreen, Zones (6) 7-9. Dioecous (male and female plants).  This long-maligned plant is making a comeback.  I prefer the simple elegance of the solid green forms, but there are also some lovely, variegated selections.   It can handle light to fairly dense and dry shade; it will burn in the sun in hot areas.  Some clones will display fleshy red berries throughout the winter.   

‘Rozannie.’ Solid green compact form that is self-sterile. It produces large quantities of Bright Red fruit.    

‘Picturata.’ Solid gold leaf center.  

Illicium floridanum.  U.S. Native, Florida to Louisiana — to wet swampy areas, adaptable to average soils.  Zones 5-9, partial to full shade.  Suckering shrub.   I read an ID trick: bruised foliage smells like gin and tonic!  Foliage and fruit is poisonous to livestock. ‘Halley’s Comet,’ ‘Florida Sunshine.’  

Danae racemosa- Poet’s Laurel.  Sometimes listed as a member of Asparagus family.  From Iran and Caucus regions.  Zones (6) 7-8, dry shade, partial shade.  Slow growing from seed.   Arching stems, 3’ tall by 4-5’ wide.     

Sarcococca hookeriana var humilis - “Sweet Box.”  Native to China, Zones 6-9.  Evergreen groundcover.  Tiny, extremely fragrant flowers in early spring.  

Sunny Herbaceous Layer  

Amsonia hubrichtii – “Blue Star.” Native to south central U.S. Full sun to partial shade, average to dry soils.  Blooms April/May with little blue flowers.  Can cut back to 6” after flowering to reduce plant flop.  

Salvia officinalis ‘Berggarten’  Berggarten Sage.  Selection of culinary sage, native to Mediterranean areas (Italy – Albania).  Zones 4-8, full sun, well–drained soils.   

Euphorbia characias sp wulfenii.  Southern Europe to Balkan areas, Zones (6) 7-10.  Demands well–drained soils.  Awesome in rock or gravel gardens.  

Shade herbaceous layer   

Hakonechloa macra ‘Aureola’ - Japanese Hakone Grass or Japanese Forest Grass.  Zones 5-9, partial shade to shade, moist to average soils.  Love the texture, movement and sound.    

  • macra– solid green, ‘All Gold’, and green and white forms.   

Carex oshimensis ‘Everillo’ - Golden clump-forming Japanese sedge.  Native habitat is dry woods and rocky slopes.  Best in moist shade, but tolerant of dryer soils once established.       

A slew of new great cultivars including ‘Everest’ – green and white, ‘Eversheen’ green and yellow.   

Also ‘Ribbon Falls’ - solid green and ‘Feather Falls’- green and white variegated.   

Q & A: 

Q–Could you identify which plants are native?  
A–I highlighted all the U.S. natives in red in the list above.  Note: some studies have found U.S. native selections for colored foliage are not as attractive as the “normal” green leaf to insects in their larval stage.  

Q–Is edgeworthia deer proof?
A–I (fortunately!) have no experience with deer and try to speak only from my experience.  Many people have stated it is not a deer favorite, but I cannot offer any personal knowledge.  

Q–How does cooking salvia do in overwintered pots? 
A–I have found no problem overwintering Salvia officinalis in a pot in my hardiness zone.  In colder zones, I would be sure I planted it in a large, insulated container with well–drained soil. The reason for a large, insulated container is to help mitigate freeze/thaw fluctuations which increase dramatically with a small soil mass. I would also place this container where it gets plenty of sunshine, but out of harsh winter winds.  One thing to remember with all containers is to check their soil moisture level occasionally during the winter.  With evergreens (and evergreys!) the plant is still in active growth and needs moisture.    

A–The sage Janet highlighted will overwinter if grown in zones warmer than D.C’s. The roots freeze and thaw too often to thrive in our area. The salvia does get woody as it ages, but don’t cut it back until you see new growth on the plant in the spring.  – Cindy Brown, Smithsonian Gardens’ Collections, Education and Access Manager 

Q–My sage plant is huge and gets very woody.  Is it ok to cut it way back?  
A—Absolutely, though the critical point is WHEN to do the deed. Certain semi-woody plants (examples: Artemisia (wormwood), Salvia officinalis (Sage), Lavender, Rosemary and Perovskia (Russian Sage)) will be killed if pruned while not in active growth. Sit on those pruners until you see abundant new growth in the spring.  Only when you have healthy new growth can you start pruning. Remember to leave at least some of the new growth when you are pruning!   Shaping and selective pruning can be done throughout the summer while the plant is in active growth.   

Q–How weedy (aggressive) is the Euphorbia?
A–Many species of Euphorbia can frolic through the garden quite a bit.  I have a love/hate relationship with Euphorbia wallichiana which pops up in unexpected places.  I love the chartreuse flowers, but that species is a rebloomer, so it blooms and sets new seed frequently.  On the other hand, the Euphorbia characias ssp wulfenii I spoke about only blooms once a year, and as long as you remove the seed heads before the seed ripens, there is absolutely no self–sowing.  Hybrid selections of Euphorbia are sterile and will not produce seed.  

Q–The red marigolds next to the Euphorbia? Fabulous!
A–Thanks—I got that as ‘Cottage Red’ but there is one on the market called ‘Glowing Embers’ which is close and has the same loose, airy growth.   

Q–I admire the wisteria blossoms in the Smithsonian Gardens.  How do you get it to bloom?  
A–Most likely you are admiring the Wisteria sinensis at the National Museum of American History; there are also glorious specimens at the National Gallery of Art.  These Asian species of Wisteria are glorious, but in the wrong hands they have become an invasive species on the U.S. east coast, climbing 80’ or more and overtaking trees.  Without proper and diligent pruning, Wisteria sinensis is like Chlorophyllic Boa constrictor climbing whatever it can find and strangling its host.   The plants you see in the Smithsonian gardens are old plants that have been heavily pruned and controlled for decades.  (Wisteria are known to not even bloom until they are at least a decade old.)    

If you must plant a wisteria, may I suggest planting our native Wisteria frutescens?   The floral displays are smaller but it is a repeat bloomer.  It is still a vigorous vine, however, reaching 40’ if left unpruned.   Honestly, I would just enjoy Smithsonian Gardens’ wisteria and save yourself, and the next generation, a LOT of work! 

Q—I’ve heard from colleagues that H.macro tolerates more light.
A–Yes, I would agree. Solid green foliage can normally handle more intense sun than the same type plant with some variegation (less chlorophyll).  Many plants that need shade from our intense summer sun flourish in full sun in other parts of the world.  It all depends on the intensity of the sun and soil moisture levels.  {By intensity of sun, I am thinking of the comparison of full sun in Arizona verses full sun in Vancouver.}  

Q–When is the best time to divide amsonia?  
A–Ha ha!  When you have someone young and strong willing to help!   Seriously, this is not an easy task on large older specimens–I have broken a few shovels doing this.  I would attempt it in the fall in my area.  Cut the stalks back so you can see the base, then using an ax or something similar, attempt to cut it into pieces.  Also dig a trench around the base and try prying off pieces.    

Q– You have some interesting hellebores there. What are their names?  
A–I have about 20 different species and cultivars in the Ripley Garden, ranging from the old fashioned Helleborus x hybridus which has downward facing flowers, to many of the newer hybrids with outward facing flowers (examples: Glenda’s Gloss, Molly’s White, Yellow Lady), to species such as H. nigra, H. argutifolius, H. nigra and lots of hybrids in between.  I love to show people the diversity within a family.  So often I hear visitors saying ‘they know’ hellebores if they have grown one type.  That is akin to saying you know the Smith entire family because you met George Smith—rubbish!  

Q–What was the green Hellebore with the Ribonfalls? 
A–Helleborus foetidus – one of my favorites!  It has the unfortunate common name of “stinking Hellebore” — so wrong! It is a sweetie that handles dry shade and, if allowed, will self sow to form a mass.  If you have enough, simply cut off the flower heads before the seed ripens.  

Q–Does the Smithsonian also have a native plant garden?  
A–Yes – actually two!  The Pollinator Garden adjacent to the National Museum of Natural History and the landscape around the National Museum of the American Indian.   

Q–What Zone are you in D.C.?  
A—D.C.‘s Plant Hardiness Zone is listed as a USDA 7a, however the Ripley Garden is protected on both sides by buildings and lots of pavement that radiates heat all around, so it is a bit warmer than that.  What I have found over years of gardening is that I lose more plants to winter wet than to cold temperatures.  When I want to try marginally hardy plants, I will make sure to plant them in the spring so they have a full season to get established before the first winter test. 

 Q—What works best for a rain garden?  
A–All of the following should thrive in a rain garden: Physocarpus, Illicium, Amsonia, Hakonechloa and carex.  

Q–I think Janet could do a second presentation on the topic with different plants.  She has taken her decades of experience and put a spotlight on useable plants that grow without constant care.
A–Thank you for the vote of confidence… I am sure you will hear more from me in the future!